1. Technical Field
This invention relates to the repair of worn or damaged refractory linings and, more particularly, to a method of and apparatus for flame spraying refractory materials containing chromium, aluminum and/or magnesium oxidizable particles for in situ repair of these linings.
2. Description Of The Related Art
Metal processing furnaces, ladles, combustion chambers, soaking pits, and the like are lined with refractory brickwork or coating. These linings become eroded or damaged due to the stresses resulting from high temperature service.
It has long been the objective of operators to repair such ovens or furnaces linings in situ while they are hot. Such in situ repair eliminates the need for cool down and heat up time periods, as well as thermal shock damages caused by excessive temperature change.
The technique of flame spraying is well known in the art. By this technique, molten or sintered refractory particles are sprayed from a lance into the furnace under repair. Such a lance may be wrapped in a fiber protective blanket or may be provided with a water cooled outer jacket so as to protect it from the high temperatures encountered during the spraying operation.
Previous flame spraying techniques used pulverized coke, kerosene, or propane gas as a fuel which was mixed with refractory powders and oxygen, and projected against the wall being repaired.
British Patent Specification No. 1,151,423 teaches entraining powdered refractory in a stream of fuel gas. Patent Specification No. 991,046 discloses entraining of powdered refractory material in a stream of oxygen, and using propane as a fuel.
U.S. Pat. Nos. 2,741,822 and 3,684,560 and Swedish Patent No. 102,083 disclose powdered metals as heat sources. These processes allow the formation of shaped masses of refractory by oxidation of one or more oxidants such as aluminum, silicon and/or magnesium in the presence of refractory oxides such as Al.sub.2 O.sub.3, MgO or SiO.sub.2. These processes teach the use of finely divided, oxidizable metal powders having a size below about 50-100 microns. This size oxidizable metal promotes rapid oxidation and evolution of heat so as to liquify or soften the entrained refractory particles as well as to soften the area being repaired. It is taught that these processes are dangerous due to flash-backs. During a flash-back, the reaction can travel back up the lance or the carrying hose to the machine or the operator, and can cause injury as well as disruption of the repair. Flash-backs are a major disadvantage of flame-spraying processes.
British patent application No. GB2035524B teaches a process wherein a carrier gas of air or other inert gas is used to convey a powdered refractory and oxidizable substances to the outlet of a lance where they are mixed with oxygen which was separately conveyed to the outlet of the lance. While overcoming some of the hazard of flame spraying refractory and oxidizable powders, this process results in extremely low deposition rates. The low deposition rate is due to the small quantity of mixture carried in the inert gas, about 0.5 kg in 50 to 100 liters per minute. The large amount of oxidant necessary to overcome that proportion of air adds to the expense of the process and introduces further dangers, such as occur when the materials are mixed together. For instance, the example teaches the use of 40% of metal oxidants in a -100BS mesh form (about 150 microns). This process also consumes very large volumes of oxygen to offset the inert gas carrier in a ratio of about 2:1 to 4:1.
The flame spraying of refractory oxides of aluminum, silicon, and/or magnesium is well known in the art. But when silicon and aluminum/magnesium are used as fuels in conjunction with these refractory oxides, residual silicon (SiO.sub.2) is produced so that the resulting deposited refractory masses are not sufficiently refractory to withstand the wear and tear of high erosion environments. Oxidizable powders and refractory powders which would yield more wear resistant deposited refractory masses, such as chromium fuel to deposit residual chromium oxide, and zirconium fuel to deposit zirconia, are highly reactive and have heretofore not been usable in flame spraying methods due to backflashes, etc.
It would be desirable, therefore, to have a method of and apparatus for flame spraying entrained refractory and oxidizable powders which achieves significantly higher deposition rates than obtainable in the past, as well as which allows for the use of oxidizable and refractory powders which, up to now, have been deemed too reactive and too prone to induce back-flashing and large system explosions.